In some cases, one of the functions sought after in a mobile station is the ability to search for another carrier (i.e., a wireless network operated by a communication service provider) while the mobile station is communicating with a specific carrier. For example, the mobile station's contracted carrier signal might not be found at the current location, but an accessible roaming carrier might exist.
Communicating wirelessly via roaming incurs more costs than the originally-contracted communication charges. For this reason, the mobile station may also search for a contracted carrier signal at a fixed time interval while connected (coupled) and communicating on a roaming carrier. When it finds the contracted carrier, the mobile station then re-connects (re-couples) to the contracted carrier.
To execute such a carrier switching control, the mobile station typically attempts to synchronize to a known or unknown frequency and acquires network configuration information regularly broadcast from the base station via a synchronized base station.
Network configuration information is information indicating the Network Service Providers (NSPs) to which the carrier (also referred to as the Network Access Provider (NAP)) is connected. The NSPs provide various Internet services to the user. Via the base station, the mobile station acquires such network configuration information, and determines whether or not the carrier is appropriate for connection.
Meanwhile, a broadband wireless access system known as WiMAX (Worldwide Interoperability for Microwave Access: IEEE 802.16e 2005, 802.16 Rev2 D8, WiMAX Forum WiMAX End-to-End Network Systems Architecture (Stage 3: Detailed Protocols and Procedures)) exists. WiMAX defines a protocol known as Network Discovery (hereinafter abbreviated as NWD), which stipulates the procedure by which a mobile station searches for a carrier and connects to a desired carrier as described above.
The NWD procedure involves the mobile station successively attempting to synchronize with a frequency registered in advance, or with all frequencies in a radio frequency (RF) profile. If synchronization is achieved, the mobile station receives a Service Identify Information Advertisement (SII-ADV) message (herein corresponding to the network configuration information) regularly broadcast by the carrier.
An example of the related art in carrier-switching technology is given in International Publication WO2006/055716.